High vacuum pumps



Nov. 21, 1961 Filed July 2, 1958 P. GARIN, ET AL HIGH VACUUM PUMPS 4 Sheets-Sheet J.

MECHANICAL PUMP lNVENTOR PAUL GAIUN ml P/EKIQE pKUGI/fi BY 97; 4am

ATTORNEY Nov. 21, 1961 P. GARIN EI'AL HIGH VACUUM PUMPS Filed July 2, 1958 4 Sheets-Sheet 2 NVEN TOR BY 57443 ATTORNEY Nov. 21, 1961 P. GARIN ETAL 3,009,629 HIGH VACUUM PUMPS Filed July 2, 1958 4 Sheets-Sheet 3 Nov. 21, 1961 P. GARIN ETAL 3,009,629 men VACUUM PUMPS Filed July 2, 1958 4 Sheets-Sheet 4 United States Patent 3,009,629 HIGH VACUUM PUMPS Paul Garin, Clamart, and Pierre Prugne, Gif-sur-Yvette,

France, assignors to Commissariat a IEnergie Atomique, Paris, France, a state administration Filed July 2, 1958, Ser. No. 746,256 Claims priority, application France July 5, 1957 7 Claims. (Cl. 230-69) Known pumps for obtaining high pumping speeds at low pressure, for all gases, belong tothe following groups:

Pumps making use of liquids such as mercury or suitable oils having a low vapour tension; in such pumps. the vapour tension of the liquid that is used limits the final vacuum that is obtained; means (often constituted by liquid air traps) are generally provided to prevent the mercury (or oil) vapour from entering the space to be evacuated; such means are described for instance in pages 214 and 215 of the book of S. Dusham (Scientific Foundations of Vacuum Technique, Ed. Wiley & Sons, New York 1949);

Pumps making use of the getter effect of a vaporizing metal such as titanium, pumps making use of the ionic pumping effect of gases by means of a suitable ionisation system placed in the space to be evacuated or again pumps making use of both of these effects. A pump using simultaneously a getter effect and an ionisation effect is described in the British Patent No. 768,003 published on February 13, 1957. Experience teaches that if the flow rates obtained with such apparatus for hydrogen, nitrogen, oxygen and organic gases are important, they are from 7 to 8 times lower for air and for rare gases. It seems that only the chemically active gases are absorbed by titanium, rare or inert gases being pumped by ionisation and at a very low rate.

By raising the total consumption of the ionisation system tooabout 3 kw., it has been possible to raise the pumping speed for argon from 9 liters per second to 160 liters per second and even 250 liters per second with an ionisation current of 1 ampere. The electronic apparatus required for the obtainment of such ionisation currents is very voluminous and delicate.

The object of the present invention is to provide a getter pump including a low temperature trap and making it possible to obtain high pumping speeds for all gases (even for rare gases) at low pressure.

This pump is essentially characterized in that high pumping flow rates for all gases at low pressure, either chemically active or not, are obtained by combining the getter effect due to a vaporized metal, sensitive to chemically active gases, and the condensation of the neutral gases on the cold wall of a very low temperature trap.

A pump according to this invention therefore includes, in combination, means for vaporizing a getter substantially in continuous manner and means for condensing the gases that have not been extracted by the getter.

A preferred embodiment of the invention will be hereinafter described with reference to the accompanying drawings, given merely by way of example and in which:

FIG. 1 is a diagrammatic axial section of a pump made according to the invention.

FIG. 2 is an axial section of the vaporizing crucible mounted on the high voltage electrode and forming a portion of the getter vaporizing device of the pump of FIG. 1.

FIGS. 3, 4, 5 and 6 show, in logarithmic coordinates, the flow rates plotted in ordinates in liters per second of the pump of FIGS. 1 and 2, respectively for air, nitrogen, oxygen and hydrogen, the pressures being plotted in abscissas, in millimeters of mercury.

Referring first to FIG. 1, the pump includes a pump body C constituted by a metal plate 1 and a cylindrical ICE casing 2 fixed thereon and cooled by circulation of water in conduits 3.

At the centre of plate 1 a high voltage electrode 4 (to be more fully described hereinafter) supports a crucible 5 made of a refractory material such as tantalum carbide. A thermionicfilament 6, running around crucible 5 and supported by two low voltage electrodes 7 is heated by Joule effect.

A mechanical feed system (diagrammatically illustrated by its casing 8) of a known type (including for instance two driving rollers rotating in opposed directions), fixed under plate 1 on the outside of the pump body C, ensures, through a guide 9, the continuous feed of a metal wire 10 along the axis of crucible 5. The feed mechanism 8 is located on the outside of the pump body so as to be protected against the depositing of condensed metal.

A screen 11, constituted by concentric cylinders of thin sheet metal prevents the vaporized metal of the getter from condensing in the chamber E to be evacuated, without substantially reducing the efiiciency of the pump. This chamber E is fixed on casing 2.

'A circular hole 13 in plate 1 and a passage D provide a communication between the inside of the pump body C and the liquefied gas trap P.

This trap includes, on the one hand, a cylindrical outer casing 14 in which is placed the vessel 15a which contains the liquefied gas 15, thermally insulated and precooled by suitable means constituted for instance by a liquid nitrogen jacket 16 and, on the other hand, a system of screens 17 placed at the temperature 'of jacket 16. Vessel 15a is fed with liquefied gas through tube 18.

Jacket 16 and screens 17 are disposed in such manner as to provide a sufficient passage for the flow of gas toward an intermediate zone F evacuated by a preliminary pumping system constituted for example, by a mechanical pump G. A metallic sealing valve 19 for molecular vacuum, located at thelower part of trap P, permits of insulating the pump system C, D, P, F and the chamber E to be evacuated from this preliminary pumping tem G.

Two helical bafiles 20 and 21, located respectively in passage D and in the passage 22 for the evaporation of the liquefied gas 15 from vessel 15a, placed at the temperature of liquid nitrogen, limit the transmission of heat by radiation from the getter pump unit to trap P.

Gas-tightness between the various pump elements (C, D, P, F) and bet-ween adjoining elements (E,"19) is ensured, on the one hand, by a system of two concentric toroidal seals, one of which, 23, is metallic and placed on the inner side and the other, 24, made of rubber, is placed on the outer side and, on the other hand, by an intermediate pumping in the annular space 25 between the two seals.

FIG. 2 shows that the high voltage electrode 4 includes a steel rod 4a, surrounded by a tube 26 of steatite and a tungsten rod 4b carried by rod 4a. Tube 26 is fixed on rod 4a by a brass cup shaped member 27 brazed at 28 on rod 4a and welded at 29 to tube 26 itself.

A brass ring 30, welded at 31 to the steatite tube 26, is provided with two grooves 32 and 33 in which are housed respectively a metallic seal 34 and a rubber seal 35, a connection 36 making it possible to establish a vacuum between these two seals. A flanged member 37, applied by peripheral screws not shown, flattens seals 34 and 35 and keeps rod 4a on plate 1.

The refractory material crucible 5 is fixed to the end of thetungsten rod 412 by means of a graphite sleeve 39 and of pin 40.

A cap 41 made of lava protects steatite tube 26 against the depositing of condensed metal thereon and a metallic casing 42 surrounds the lower portion of electrode 4a, 4b,

thus limiting discharges when the heating of crucible is started.

'It will be supposed that the pump made according to the invention has the following characteristics given merely by way of indication: the vaporizing metal is titanium; the source of cold of the trap is liquid helium, the boiling temperature of which is 4.2 K.; the temperature of chamber E is close to 300 K., that of jacket 16 close to 100 K. In such a pump, the rate of vaporization of titanium, which depends upon the rate of feed of the metal wire may be adjusted from 2 milligrams per minute to 20 milligrams per minute.

During the operation of this pump, the free end a of the metal wire 10, driven by the feed device 8, is moved downwardly in a continuous manner and comes into contact with crucible 5 brought to the suitable high temperature. As a consequence of this contact, the wire is vaporized and the metal vapours are condensed on the cold wall of easing 2.

When condensing on the wall of easing 2, titanium fixes the chemically active gases such as hydrogen, oxygen and nitrogen.

The rare gases, which are but little fixed by titanium, condense upon the cold wall of the liquid helium trap (of course with the exception of helium). These rare gases form only one percent of the composition of air, so that the pumping speed of the trap must be equal to at least one hundredth of the rate of pumping of the whole of the pumping system.

As the percentage of helium in air is very low, a low rate of ionic pumping is sufiicient to pump these rare gases. The device for heating the crucible constitutes in itself an ionisation pumping system which is quite sufficient to eliminate the helium present in air.

The geometry of the system including the chamber E to be evacuated, the titanium pump C and the trap P is such that air or another gas pumped from chamber E first passes through titanium pump; therefore, the gases coming into contact with liquefied gas are mostly rare gases and in particular argon.

The results which will be hereinafter given have been obtained with a pump making use of titanium and of liquid helium, of the type shown on FIGS. 1 and 2, having the above cited characteristics and also the following characteristics:

Diameter of the titanium wire: 0.5 mm. Nature of the crucible: tentalum'carbide Temperature of the crucible: 2000 C.

On each of FIGS. 3, 4, 5 and 6, where the scales are logarithmic,

Curve I shows the pumping rate as a function of the pressure, the measurements being made from the initial vacuum obtained in chamber E which is little degassed and Where there are still some micro-leaks,

Whereas curve II shows the pumping rates from the final vacuum obtained in the chamber which has been better degassed and the gas-tightness of which ha been improved.

FIG. 3 shows a curve III giving the pumping rates for air when only the titanium vaporizing system is in operation and the liquefied helium trap is not being used (only the chemically active gases being mainly pumped). The comparison with the curves of FIG. 4 which relate to notrogen shows that the ratio of nitrogen pumping speed to that of the air pumping speed for a residual pressure in the chamber to be evacuated equal to 210* millimeters of mercury is about 30 (FIG. 4 and curve III of FIG. 3). When the liquid helium trap is brought into operation, this ratio is reduced to about 1 (FIG. 3: curves I and II; FIG. 4: curves I and II).

Consequently, a pump according to the invention comprises in combination:

On the one hand, means (gettering means) which act chiefly on the chemically active gases such as oxygen, nitrogen and hydrogen, these means being capableof a 4 higher efliciency since this efiiciency depends upon the area of condensation;

On the other hand, condensation means (liquid helium trap) which pump all the other gases with the exception of helium.

The lowest pressures thatare obtained do not characterize the limit vacuum of the pump: as a matter of fact, comparison between the two curves I and II of FIG. 3, which correspond to air, and also between the two curves I and II of FIGS. 4, 5 and 6, which correspond to chemically active gases, shows that the flow rate at low pressures (which limits the final pressure in the evacuated chamber) is due only to the quality of said chamber, that is to say to its gas-tightness and to the amount of degassing of the various parts which constitute it.

It will be further noted that for very low pressures (below 10 millimeters of mercury), as the deposit of titanium requires a long time to become saturated, it is preferable to stop the evaporation of titanium and to keep in operation only the heating system (ionic pump and getter) and the liquid helium trap; thus degassing of the titanium wire itself when it is vaporized is avoided and it is then possible to obtain very high vacuums (lower than 10' millimeters of mercury).

A pump according to the present invention makes it possible to obtain for air a rate of pumping equal to that obtained for chemically active gases. Up to the present time, the known apparatus did not achieve this characteristic although they made use of a heavy electronic system to eliminate the rare gases.

In a general manner, while we have, in the above description, disclosed what we deem to be practical and efficient embodiments of our invention, it should be well understood that We do not wish to be limited thereto as there might be changes made in the arrangement, disposition and form of the parts without departing from the principle of the present invention as comprehended Within the scope of the accompanying claims.

What we claim is:

1. A high vacuum pump which comprises, in combination, a pump body, means for continuously feeding an elongated element of getter metal into said pump body, means in said pump body for heating said getter metal element as it is being fed and vaporizing it, a chamber to be evacuated in communication with one end of said body, a gas trap chamber in communication with the other end of said body, a vessel in said gas trap chamber containing a gas liquefied at low temperature, means for preventing heat transfer from the outside of said gas trap chamber to said vessel and means for continuously evacuating said gas trap chamber.

2. A vacuum pump according to claim 1, in which said vessel contains liquefied helium.

3. A vacuum pump according to claim 1 in which said means for preventing heat transfer include a jacket fed with a gas liquefied at a low temperature higher than that of the liquefied gas contained in said vessel.

4. A pump according to claim 1 including, between said pump body and said gas trap chamber, a passage of small cross-section and a helical bafile in said passage.

5. A vacuum pump according to claim 1 further comprising, between said chamber to be evacuated and the portion of said pump body adjacent thereto, a plurality of concentric cylinders for stopping the metallic vapours.

6. A vacuum pump according to claim 1 further including means forming a space adjoining said gas trap chamber and adapted to be evacuated by said evacuating means, and valve means adapted to separate, when closed, said space from said evacuating means.

i 7. A high-vacuum pump comprising a pump body with a first and a second opening, said pump body communicating through said first opening with a vessel to be evacuated, means for continuously feeding a gettering' substance in said pump body, means in said pump body for continuously vaporizing said getter-ing substance in 5 said pump body, screen means located in said first-opening for preventing a material quantity of said gettering substance, when vaporized, to reach said vessel through said first opening, a gas trap chamber communicating with said pump body through said second opening, a container located in said gas trap chamber and containing a liquefied gas at a very low temperature, a cold jacket located in said gas trap chamber, around said container, for isolating said container from heat transing means openatively connected to said gas trap chamber for continuously evacuating said gas trap chamber.

References Cited in the file of this patent UNITED STATES PATENTS Snook July 15,1924 Herb Sept. 2, 1958 

